1. Field of the Invention
The present invention relates to a condenser and a power generating installation using the condenser, wherein the condenser is used for maintaining the discharge pressure of a steam turbine.
2. Description of the Related Art
A condenser is used to condense saturated steam discharged from the final stage of a steam turbine to maintain the discharge pressure at vacuum. The condenser is provided with a number of cooling tubes. Steam led to the condenser is condensed on the outside surface of the cooling tubes. Cooling water introduced into the cooling tubes, such as the sea water, water from a cooling tower, etc. draws condensation latent heat from the steam to maintain the discharge pressure at vacuum. However, when the temperature of cooling water varies due to change in season, etc., the condensation temperature and saturated steam pressure may also change, resulting in vacuum of the condenser (condenser vacuum) fluctuating.
The output of the steam turbine changes with the vacuum of the condenser. A decrease in the vacuum (that is, an increase in the saturated steam pressure) reduces heat drop which passes through a turbine stage, resulting in reduced output of the steam turbine. On the other hand, there is a tendency that an increase in the vacuum increases heat drop and thus the output of the steam turbine is improved. However, when the axial velocity of the final stage of the turbine reaches the acoustic velocity because of an increase in the specific volume of steam, the power obtained by the turbine will be saturated. Thereafter, even when the vacuum is more increased the turbine output will improve no longer. When the vacuum is increased after the power is saturated in this way, the condensed liquid temperature falls and the heat required for re-heating increases, thus decreasing the output of the steam turbine as a system. For these reasons, there exists, in a power plant, condenser vacuum (hereinafter referred to as optimal vacuum) at which the output is maximized.
However, there has been a problem that the condenser vacuum is not necessarily maintained in such a manner that the maximum output is obtained since the condenser vacuum changes with seasonal variation of the cooling-water temperature, etc. as mentioned above. With one technique intended to solve the above-mentioned problem, the number of a plurality of cooling-water pumps under operation is changed to control the quantity of water taken from a water source (for example, the sea) to control the quantity of cooling water supplied to the condenser, thus optimizing the vacuum (refer, for example, to JP-A-2007-107761).